1. Field of the Invention
The present invention relates to a microstrip reflectarray antenna and, more particularly, to a microstrip reflectarray antenna with lower cross polarization level for operation in a satellite communication system.
2. Description of Related Art
In a conventional satellite communication system such as satellite TV, the available operation frequency range of the channel signal transmission is highly restricted by the absorption of the atmosphere or other related factors. Currently, as the number of the channels to transmit channel signals increases significantly, i.e. hundreds of cable TV channels, the conventional satellite communication system, which uses different frequencies to transmit different channel signals (i.e. frequency multiplexing method), is no longer sufficient for operation. As a result, another conventional satellite communication system, which uses a plurality of same frequency signals having different polarization directions to transmit different channel signals, is then proposed. By using this satellite communication system employing the frequency multiplex method, the channels available for transmitting the channel signals can be increased significantly. As a result, there is not immediate need to launch new satellites, which results in saving a huge amount of money.
As described above, in the aforesaid frequency multiplex method, several channel signals share the same channel to transmit and receive by the antenna. Thus, if the antenna of the conventional satellite communication system cannot clearly recognize the polarization direction of the channel signal it is designated to receive and filter out the channel signals with other polarization directions, the antenna of the conventional satellite communication system may receive two or more channel signals at the same time. Although the strength of the channel signal (target signal) is higher than the other channel signals (noise signals), the reception of the target signal will still be influenced by the noise signals. FIG. 1A shows a schematic diagram of the microstrip reflectarray antenna of the prior art. The microstrip reflectarray antenna of the prior art comprises a ground plate 11, a reflecting plate 12, four supporting units 13 and a horn antenna 14. The reflecting plate 12 is supported by the four supporting units 13 being composed of the insulating materials, and thus a predetermined distance between the reflecting plate 12 and the ground plate 11 being composed of copper is maintained. Besides, the microstrip reflectarray antenna of the prior art further comprises a plurality of microstrip antenna units 15 locating on the upper surface 121 of the reflecting plate 12. Each of the microstrip antenna units 15 comprises an inner ring 151 and an outer ring 152. Furthermore, the size of each of the microstrip antenna units 15 corresponds to its location on the upper surface 121 of the reflecting plate 12. Moreover, these microstrip antenna units 15 of the microstrip reflectarray antenna of the prior art further comprise some characteristics as described as below:
1. There is a predetermined ratio relationship between the length of the second diameter of the inner ring 151 and the length of the first diameter of the outer ring 152 of the same microstrip antenna unit 15.
2. Both the outer ring 152 and the inner ring 152 of the same microstrip antenna units 15 have the same width (4 mm).
FIG. 1B shows a schematic diagram of the IE3D software simulation result of the plane wave scattering field of the microstrip reflectarray antenna of the prior art. As shown in the figure, the cross polarization level (XPL) of the microstrip reflectarray antenna of the prior art is remarkably high. Therefore, when the microstrip reflectarray antenna of the prior art is designated to receive a high frequency signal with Y-polarization direction, the microstrip reflectarray antenna of the prior art can still receive some high frequency signals with X-polarization direction at the same time, with the noise degenerating the reception of the high frequency signal with Y-polarization direction.
Therefore, a microstrip reflectarray antenna which can receive a high frequency signal with single polarization direction, such as a microstrip reflectarray antenna with lower cross polarization level, is required in the field, so as to increase the reception quality and the number of available channels of a satellite communication system.